1. Field of the Invention
The present invention relates to unsaturated gas plants for either fluid catalytic conversion or thermofor catalytic conversion systems.
2. Background of the Invention
In conventional unsaturated gas plants, the compressor aftercooler acts in a similar manner to a partial condenser to the stripper. This causes excessive recycle between the low temperature separator and the stripper. Also, because all unstabilized gasoline enters the absorber, excessive light ends recycling occurs between the low temperature separator and the absorber.
The conventional unsaturated gas plant, shown in FIG. 1, shows low pressure gas from, e.g., a fluid catalytic conversion (FCC) main column overhead accumulator being fed to a first stage compressor 1, while unstabilized gasoline from the main column overhead accumulator is fed to primary absorber 3. The gaseous output from first stage compressor 1, which was heated in compressor 1, is fed to interstage cooler 5 which cools this gaseous output so that the gas going to second stage compressor 9 will be at a lower temperature to provide increased energy efficiency. The mixed output of interstage cooler 5 comprises a mixed-phase liquid and gaseous portion which is sent to interstage receiver/separator 7 which provides a gaseous output to second stage compressor 9 and a liquid output along line 11. Line 11 also contains water wash to the unsaturated gas plant. Gaseous product discharged from second stage compressor 9 is combined with bottoms product from primary absorber 3, stripper overhead from stripper 13 and liquid output from interstage receiver/separator 7 to form a mixed product in line 25 which is fed to aftercooler 17. The output of aftercooler 17 is fed to low temperature-high pressure separator 15 where it is flashed and water is separated from the hydrocarbons. Liquid hydrocarbon bottoms product from separator 15 is fed to stripper 13 and vapor from high pressure-low temperature separator 15 is fed to primary absorber 3. Bottoms product from stripper 13 is passed to a debutanizer, while stripper 13 overhead product is sent along line 19 to mix with lines 11, 21 and 23 prior to being fed to aftercooler 17, as noted above.
The FIG. 1 prior art system is not energy-efficient due to mixing of the hot gas outputs from second stage compressor 9 and stripper 13 and the relatively cool liquid outputs from interstage receiver/separator 7 and primary absorber 3 bottoms product. After mixing, these gaseous and liquid outputs are sent through aftercooler 17 prior to being fed to three-phase low temperature-high pressure separator 15. Input line 29 into separator 15 carries a mixed stream at relatively low temperature. Because of this relatively low temperature, the heavy ends in line 29 absorb a relatively large amount of light ends. Thus, the bottoms product of low temperature-high pressure separator 15 contains a relatively large amount of light ends, requiring stripper 13 and its reboiler 31 to be oversized and to recycle light ends between stripper 13 and low temperature-high pressure separator 15 continuously via stripper overhead line 19.